Forward pull on a sail is caused by air flowing over its surface. To understand this concept, compare your sails to the wings of an airplane. Lift is required to keep a plane in the air, and lift is required to keep a sailboat moving forward.

On an airplane, air splits and passes on either side of its wings. The air on the upper side of a wing has greater velocity than the air on the lower side - because of the angle of attack and an airflow phenomenon called circulation effect. The angle of attack is defined as the angle between the chord (a line from the leading edge to the trailing edge) of an airfoil, and a line representing the undisturbed relative airflow. The circulation effect causes a circular flow that affects velocity and reinforces and accelerates airflow on the upper side of the wing (the leeward side of the sail), while the velocity of flow over the under side of the wing (the windward side of the sail) is opposed and decreased.

Unlike an airplane's wing, all lifting surfaces of a sailboat-the keel, hull, centerboard, and rudder - are symmetrical. Yet they can still develop lift because water hits them at an angle. As air flows past a sail, the sail's curve causes the flow to bend. On the back side of the sail - the leeward side - this results in a greater distance for the wind to travel. In 1738, Daniel Bernoulli discovered that as velocity increases pressure decreases, creating a lift that acts at right angles to the surface. When velocity on both sides of the sail and the difference in velocity between the sides of the sail both increase, so does lift.

Deflection of flow is an action that causes an equal and opposite reaction according to Newton's third law of motion. Do you remember, as a child, what happened when you stuck your hand out a car window and then tilted it upward? Because of this deflection and circulation flow, a flat surface or even a thin membrane such as a sail can create lift.

The air, however, has to flow over the surface smoothly and evenly. Once the air starts to separate from the surface it becomes turbulent. Instead of even flow, burbles develop that reduce suction. Much of the turbulence is caused by the angle that the airfoil makes with the airflow. This too is called the angle of attack or angle of incidence.

If the angle is small, the airflow remains attached to the surface for quite a distance back toward the leech of the sail, or the trailing edge of an airplane's wing. When the angle is increased, the airflow detaches earlier and turbulence starts to occur in the forward part of the sail. At a certain angle and speed there is so much separation of flow that the wing or sail no longer develops enough lift and a stall occurs. In an airplane the result is dramatic, since the aircraft drops suddenly. A sailboat, however, will just heel over more and slow down.

A sail stalls if it is trimmed in too tight. But a stalled sail can look the same as a sail operating at maximum efficiency. You can easily learn to trim sails properly, by easing them to the point just before they luff. A luff is easy to see because the leading edge of the sail flaps or flutters. So when trimming your sails, follow this basic rule: ease the sail until it luffs, then trim it just enough to stop the luff.

Like everything else in this world, there are some exceptions. After you have sailed for a while you may find, especially on reaches, the need to trim a little past this point to get maximum drive from the sail. This judgment depends a great deal on wind strength. In lighter winds you can trim tighter before separation and turbulence occur. But the tighter you trim, the more sideways the driving force will be; this can result in detrimental heeling rather than greater forward motion.

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